This artist's rendering provided by NASA shows the positions of the sun, Earth and Mars, at left. Radio communications between Earth and Mars are limited during this planetary alignment, which occurs in April.

(AP Photo/NASA/JPL-Caltech)

LOS ANGELES -- It's the Martian version of spring break: Curiosity and Opportunity, along with their spacecraft friends circling overhead, will take it easy this month because of the sun's interference.

For much of April, the sun blocks the line of sight between Earth and Mars. This celestial alignment - called a Mars solar conjunction - makes it difficult for engineers to send instructions or hear from the flotilla in orbit and on the surface.

Such communication blackouts occur every two years when the red planet disappears behind the sun. No new commands are sent since flares and charged particles spewing from the sun can scramble transmission signals and put spacecraft in danger.

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Mission teams prepared by uploading weeks of scaled-back activities beforehand.

"They're on their own," said Rich Zurek, chief Mars scientist at the NASA Jet Propulsion Laboratory.

The rovers are banned from driving. Instead, they take a staycation and study their surroundings. The orbiting Mars Odyssey and Mars Reconnaissance Orbiter continue to listen for the rovers and make their own observations, but for the most part will transmit data once Mars is in view again.

Opportunity, Odyssey, Mars Reconnaissance Orbiter and the European Space Agency's Mars Express have survived previous bouts of restricted communications. It's the first for Curiosity, which landed last year near the Martian equator to hunt for the chemical building blocks of life.

Beginning Thursday and through May 1, Curiosity can only check the weather every hour, measure radiation and look for signs of water below the desert-like surface. The limited chores are a departure for the active six-wheeler, which is used to driving, drilling and zapping its laser at rocks.

Before the sun got in the way, Curiosity made its biggest discovery yet: From a drilled piece of rock, it determined that its crater landing site was habitable billions of years ago, possessing some of the basic ingredients necessary to support tiny microbes.

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Scientists must wait until next month to drill into another rock and start the long-delayed trek to a mountain where Curiosity will search for the elusive organic molecules that are fundamental to life as we know it. The road trip was supposed to have started last year, but longer-than-expected science experiments put Curiosity behind schedule.

Odyssey, circling Mars since 2001, has experienced half a dozen blackout episodes with no problem. This time, it will try something new. There are plans to radio Earth every day even if calls are dropped, mostly to keep engineers updated on Curiosity's health. The rover is also programmed to send daily beeps to ground controllers.

By contrast, the Mars Reconnaissance Orbiter will record and store information onboard its computers and beam it back after the hiatus. Opportunity, which parked itself in a clay-rich spot, will use the down time to study a rock and track the amount of dust in the sky.

With Mars missions on autopilot, many scientists and engineers planned to take vacation while a small crew remains on duty.

"We've been through this before," Zurek said. "We're not expecting to have any problems during this period. Let's hope it stays that way."

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